The present invention relates to an apparatus and method for cutting tendons, especially post-tension tendons used in concrete structures, by using an acetylene torch or a plasma cutting torch.
For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is extremely weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel tendon, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly in such post-tensioning operations, there are provided anchors for anchoring the ends of the tendons suspended therebetween. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may become exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If this occurs, and the exposed portions of the tendon or anchor suffer corrosion, then they may become weakened due to this corrosion. The deterioration of the anchor or tendon can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of by-products from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
Several U.S. patents have considered the problem of anchor and tendon corrosion. For example, U.S. Pat. Nos. 4,896,470 and 5,072,558 disclose tendon tensioning anchor systems in which the metal anchor for the system is encapsulated in plastic and has a tubular portion extending outwardly towards the surface of the post-tensioned concrete body. A sealing cap is fitted to the end of the tubular portion of the plastic encapsulation to provide a fluid tight seal for protecting the post-tensioned tendon, anchor and tensioning wedges from exposure to the elements. Other prior art systems also exist in which the end of the post-tensioned tendon is severed at a point inwardly from the outer surface of the post-tensioned concrete body and means are used to protect the tendon end, anchor and tensioning wedges from exposure to the elements.
When using prior art systems for corrosion protection of the tensioning tendon and related apparatus, it is important that the tendon be terminated at a point inboard from the outside surface of the post-tensioned concrete body. This requires that the end of the tendon be cut just outboard of the tensioning wedges and within the pocket or cavity formed by the pocketformer. The most common method used in the prior art for the cutting of such tensioned tendon at this point is an acetylene torch or cutting torch. Normally, the end of the cutting torch is placed in close proximity to the face of the anchor and within the pocket. Heat is applied directly to the tendon so as to sever the tension from that portion received within the anchor. Unfortunately, the cutting of the tension with a torch at the point near the tensioning wedges can cause the tendon and wedges to become heated and can result in the loss of temper of the metal or loosening of the post-tension wedges. Alternatively, the torch is not brought into close enough proximity to the anchor such that an improper cutting of the tension occurs. In other words, the tension may be cut so that an end of the tendon extends outwardly of the pocket and outwardly of the concrete body. No techniques have been used in the past for placing such a torch in close proximity to the anchor body within the pocket without causing the torch to adversely affect the post- tensioning wedges or the integrity of the anchor.
Another technique used for the cutting of the tendon is a conventional electric saw. However, this requires that a portion of the slab or other concrete structure surrounding the anchor also be cut in order to reach the portion of the tendon which is within the pocket formed in the concrete adjacent to the anchor.
Importantly, U.S. Pat. No. 5,436,425,issued on Jul. 25, 1995 to the present inventor described a system whereby the tendon could be properly cut by using a plasma cutting torch. This method and apparatus utilized a positioning element for interconnecting the head of a plasma cutting torch with the tendon to be severed. A positioning element conforms in shape to the pocket in the concrete body adjacent to the anchor. As such, the positioning element can be placed over the tendon and pushed into the pocket prior to activating the plasma cutting torch for the purposes of severing the tendon. The plasma cutting torch utilizes an air compressor, an electrical power source and electrodes to generate a stream of air, nitrogen or other gaseous plasma at very high temperatures of about 3000xc2x0 C.-5000xc2x0 C. or more. The stream of superheated gaseous plasma cuts through the metal more quickly than in an acetylene torch. The positioning element included a combination clamp and shield member which is releasably engagable with the tendon. The clamp included an attachment means or bracket which is engagable with the cutting head of the plasma torch. The attachment bracket is appropriately sized so as to be friction fitted around the cutting head or could be permanently attached to the cutting head. The positioning element is attached to a bracket and is releasably engagable with the tendon to be cut for positioning the cutting tip a predetermined distance from the longitudinal axis of the tendon. The positioning element includes a pair of depending jaws pivotally attached to a mounting bracket and biased towards a closed position by a spring. Each of the jaws has a semi-circular relieved portion at its inner edge which, when together, provides a circular opening through which the tendon may be received. Once the tendon is received in the center opening, the jaws close upon it.
Unfortunately, in this prior art patented device, it was found that typical construction environments do not utilize plasma cutting torches. These plasma cutting torches are very expensive and are typically not operated in a proper manner by the construction workers. As such, strong resistance to the use of such a plasma cutting torch has occurred. Conventionally, construction workers continue to utilize acetylene torches for the severing of the tendon.
Another problem associated with the use of torches for the cutting of tendons associated with plastic-encapsulated anchors used in a post-tension anchor system is that the cutting torch can impart heat to the tendon to such a sufficient degree as to cause a melting of the encapsulation. Since it is important to maintain the post-tension anchor system in an encapsulated condition, all of the plastic components of such an encapsulated system must be properly maintained. If, for example, the cap-receiving portion of the encapsulated anchor should become deformed or melted by heat, it would be impossible to attach the necessary cap or sealing devices so as to assure that the post-tension system is properly sealed. As such, it is important to be able to avoid the melting of the plastic encapsulations.
In an effort to overcome problems associated with U.S. Pat. No. 5,436,425, the present inventor made an improvement in the cutting torch, as shown by U.S. Pat. No. 6,040,546, issued on Mar. 21, 2000. In this device, the cutting torch apparatus included a first shield having an opening formed therein and adapted to allow the tendon to pass therethrough, a tubular extension extending outwardly of the first shield at the opening, and a bracket member affixed to the first shield and adapted to attach to the cutting torch such that the cutting torch resides on a side of the first shield. A second shield is connected to the bracket member. The second shield has an opening adapted to allow the tendon to extend therethrough. The second shield is arranged in spaced parallel relationship to the first shield. The cutting torch has a nozzle which is interposed between the first shield and the second shield. This device was particularly effective in avoiding the igniting of grease associated with the encapsulated tendon. It further resisted the migration of any flame across the tendon toward the plastic encapsulation. The device also effectively prevented sparks from flying from the pocket.
Unfortunately, with both U.S. Pat. Nos. 5,436,425 and 6,040,546, the shields created a problem in the actual work place environment. After repeated use, the shields would become damaged or deteriorate. The deformation of such shields would prevent the device from operating effectively during the cutting of the tendon. The use of such shielding techniques was often complicated by the fact that the workmen at the construction site would not fully comprehend the proper use of such shielding mechanisms. As such, a need developed so as to simplify the technique of properly spacing the cutting torch from the encapsulation of the anchor and from the wedges of the anchor.
In certain other circumstances, the size of the pockets used with these prior art patents was too large for certain construction purposes. As such, a need developed so as to minimize the size of the pocket required for the effective cutting of the tendon. In all circumstances, the manner of creating a proper tendon-cutting method would require that the method be very simple, easy to use and easy to implement.
It is an object of the present invention to provide a method and apparatus for severing the free end of a post-tension tendon at a point near the tensioning wedges and within the depth of the pocket formed for the anchor member.
It is another object of the present invention to provide such a method and apparatus in which the tendon is cut without substantially heating the tendon and tensioning wedges.
It is still a further object of the present invention to provide a method and apparatus in which the tendon can be cut at the desired location without damaging the post-tensioned concrete body.
It is a further object of the present invention to provide a method and apparatus in which the tendon can be cut by using a conventional acetylene torch.
It is a further object of the present invention to provide such a method and apparatus which is easy to use, relatively inexpensive, easy to implement and simple to manufacture.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.
The present invention is a cutting torch for use in severing tendons used in post-tension construction. This cutting torch comprises a handle, a head connected to the handle and a nozzle connected to an end of the head. The head of the cutting torch has a pivot formed thereon. The pivot is adapted for engagement with a corresponding pivot point formed within the pocket of the concrete slab.
The cutting torch has the head and the nozzle extending transverse to a longitudinal axis of the handle. The pivot is aligned with the longitudinal axis of the handle. In one embodiment of the present invention, the pivot comprises a protrusion extending outwardly of an exterior surface of the head. This protrusion is adapted to pivotally engage an indentation formed in the concrete slab. Alternatively, the pivot can comprise an indentation extending inwardly of an exterior surface of the head. The indentation is adapted to pivotally engage a protrusion formed in the concrete slab.
The present invention is also a post-tension construction apparatus comprising a concrete slab, an anchor embedded in the concrete slab, a tendon fixedly received by and extending through the anchor and a pocket formed in the concrete slab and extending outwardly from one end of the anchor. The tendon has a portion extending through the pocket. The pocket has a wall extending outwardly from the end of the anchor. This wall has a pivot point formed thereon.
In this post-tension construction apparatus, a cutting torch is removably received within the pocket. The cutting torch has a pivot formed thereon. This pivot is pivotally engagable with the pivot point of the pocket. The cutting torch has a cutting nozzle positioned adjacent to the tendon when the pivot engages the pivot point. The pocket has a space around the tendon suitable for allowing the cutting nozzle to pivot angularly therein such that the cutting nozzle directs a flame across a diameter of the tendon in the pocket. The pivot point can either be a protrusion or an indentation. The pivot can either be a protrusion or an indentation matingly engageable with the shape and size of the pivot point.
The present invention is also a method of cutting a tendon extending outwardly from an end of an anchor embedded in a concrete slab comprising the steps of: (1) forming a pocket in the concrete slab adjacent to an end of the anchor; (2) forming a pivot point in a wall of the pocket extending outwardly from the end of the anchor; (3) forming a pivot on a head of a cutting torch so as to have a size and shape matable with the pivot point, (4) inserting the cutting torch into the pocket such that the pivot engages with the pivot point; (5) directing a cutting nozzle of the cutting torch toward the tendon; and (6) pivoting the cutting nozzle within the pocket such that the tendon is severed by cutting energy from the cutting nozzle.
In this method, the cutting torch can be removed from the pocket when the tendon is severed. The step of forming the pivot point include either forming a protrusion or an indentation on the wall of the pocket. Similarly, the step of forming a pivot comprises forming an indentation or a protrusion on the head of the cutting torch. In the present method, the step of forming a pocket includes the steps of (1) securing the end of the anchor against one end of a pocketformer; (2) affixing an opposite end of the pocketformer against a form board; (3) solidifying concrete over and around the pocketformer and the anchor, and (4) removing the pocketformer from the end of the anchor after the concrete solidifies.